1. Field of the Invention
The present invention relates to a variable valve train system for continuously controlling the valve driving output of an internal combustion engine.
2. Description of the Related Art
Reciprocating engines (internal combustion engines) mounted on motor vehicles are generally equipped with a variable valve train system attached to the cylinder head for continuously controlling the characteristics of at least intake valves, to control the exhaust gas emitted from the engine and lower the pumping loss.
Many of such variable valve train systems employ a variable valve actuation mechanism whereby the lift amount of at least the intake valves is continuously varied to adjust the amount of intake air. The variable valve actuation mechanism generally comprises the combination of a device for providing a valve driving output based on the displacement of an intake cam fitted on the camshaft, and a device for continuously varying the valve driving output (valve lift amount, valve opening/closing timing, valve open period, etc.) in accordance with the rotary displacement input from a control shaft (see, e.g., Unexamined Japanese Patent Publication No. 2005-299536).
There has also been proposed an arrangement using a driving force overcoming the valve reaction force to smoothly vary the valve driving output, wherein driving power output from a driving power source, such as an electric motor, is transmitted to the control shaft through a transmission mechanism including a speed reduction mechanism, such as a screw mechanism or a worm gear mechanism, so that the control shaft may be rotated with high torque (see Unexamined Japanese Patent Publications No. 2005-42642 and No. 2007-2686).
Where the transmission mechanism is employed, it is necessary that the engagement section between parts such as a gear and a lead screw should be lubricated with lubricating oil (lubricant) in order to permit smooth rotation of the control shaft. The engagement section between gears, in particular, is likely to be insufficiently lubricated because the gears are applied with large valve reaction force from the control shaft or, in a steady state, kept in a fixed orientation. Thus, the engagement section needs to be constantly lubricated with fresh lubricating oil.
Usually, therefore, the engine is provided with an additional oil supply system whereby part of the lubricating oil being supplied to various parts of the cylinder head is guided to the engagement section of the transmission mechanism.
To equip the engine with such an oil supply system, however, several oil passages leading to the oil gallery need to be formed in the cylinder head as well as in the parts forming the engagement section. Thus, the oil supply system is considerably complicated in structure and substantially increases costs.